The present invention relates to monitoring the composition of an effluent stream from processing chambers of the type that may be used in semiconductor wafer fabrication, and more specifically to optically detecting the composition of a plasma formed from the effluent stream from a chamber.
One of the primary steps in fabrication of modern semiconductor devices is the formation of a layer, such as a dielectric, metallic, or semiconductor layer, on a substrate or wafer. As is well known, such layers can be deposited by chemical vapor deposition (CVD), physical vapor deposition (PVD) or other methods. In thermal CVD processes, reactant gases are supplied to the substrate surface where heat-induced chemical reactions take place to produce a desired film.
In a typical plasma-enhanced CVD (PECVD) process, reactant gases are disassociated in a plasma formed by the application of energy, such as radio frequency (RF) energy to a reaction zone near or adjoining the surface of the substrate. This type of plasma is commonly called an in-situ plasma. The plasma, which contains high-energy species such as ions and free radicals, promotes formation of the desired layer. The plasma also typically produces a broad range of radiation from ultraviolet to infrared.
Deposition systems typically accumulate unwanted residue when used to process substrates. Over time, failure to clean the residue from the deposition system can result in degraded or unreliable performance of the deposition system, and in defective wafers. Increasingly stringent requirements for fabricating modern high-integration devices must be met, but conventional substrate processing systems may be inadequate to meet these requirements. Semiconductor device manufacturers may need to replace or improve their existing deposition systems in order to provide the process control and throughput necessary to compete in the manufacture of modern semiconductor devices. An example of an upgraded capability that may be required in deposition systems is the capability to clean the chamber effectively and economically in order to improve quality and overall efficiency in fabricating devices.
Typically, two types of cleaning procedures have been used. xe2x80x9cDry cleaningxe2x80x9d processes may be performed between deposition processing steps without disassembling the chamber. A dry cleaning process uses a cleaning gas or a plasma to volatilize residue in the chamber, which is then removed by the exhaust system. A dry clean may be performed after each wafer has been processed, or after several wafers have been processed. xe2x80x9cWet cleansxe2x80x9d typically involve opening the processing chamber and physically wiping down the chamber with cleaning fluids. Wet cleans can be quite time-consuming and disruptive of product flow and throughput. Periodic chamber cleaning, however, is critical to process and device performance since possible effects of residue build-up in the chamber include wafer contamination and process drift (e.g., changes in deposition rate) due to changes in chamber thermal and electrical properties. The problems of impurities and particles causing damage to the devices on the wafer are of particular concern because of the increasingly small dimensions of modern devices. Thus, properly cleaning the chamber is important for the smooth operation of wafer processing, improved device yield and better product performance.
In some instances, an in-situ plasma may be used in a dry cleaning process. While some deposition systems, such as thermal CVD systems, may not have the ability to perform this in-situ plasma clean step, PECVD systems typically have chamber components compatible with inter-deposition in-situ plasma cleans. However, the ability to form or maintain an in-situ plasma for cleaning may be limited to certain chamber pressures or other conditions, such as the type of cleaning gas used. Exposure to the in-situ plasma clean may shorten the lifetime of chamber components or degrade subsequent wafer processing. Furthermore, the efficiency of an in-situ plasma cleaning process may depend on the plasma density and distribution, and may clean some areas of the chamber differently than other areas. Therefore, it may be impractical or undesirable to use an in-situ plasma cleaning in all circumstances. Another reason to use remote instead of in-situ plasma cleaning is that emission of global-warming perfluorinated compounds (PFC""s) can be lower when using remote plasma cleans.
Remote plasma generating systems have been shown to be useful in cleaning substrate processing chambers. Some remote plasma generating systems use a waveguide to convey microwave energy from a microwave source to an applicator tube, where a precursor gas is converted into a plasma. The plasma is used to dissociate a precursor (e.g., NF3) into chemically active species, such as fluorine radicals, that are transported into the process chamber to react with the deposition residue during the clean process. Using microwave energy to generate the plasma is quite efficient, and often results in higher clean rates than are obtained with an in-situ plasma clean.
It is desirable to know when a dry clean process is complete to maximize wafer throughput, to minimize use of cleaning materials, and to minimize wear and tear of equipment being cleaned, thereby increasing the expected life of hardware and decreasing the frequency of periodic maintenance. Optical endpoint detection methods have been used to determine the endpoint of in-situ plasma clean processes. One type of optical endpoint detection system uses a photo detector to measure the light emitted by the in-situ glow discharge. When a gas is ionized, the plasma will emit radiation over a broad range of wavelengths. The intensity of the emitted radiation varies with wavelength, resulting in a characteristic distribution, or spectrum, of emission peaks versus wavelength. One can identify which molecules or other species are present in the plasma by examining the resulting emission spectrum. Remote plasma cleaning systems, however, cannot use this type of optical endpoint detection because the plasma is generated upstream of the process chamber and, hence, the observed plasma emission spectrum does not reflect changes in chamber chemistry over the course of the cleaning process.
Other methods, such as empirically defining a set xe2x80x9caveragexe2x80x9d cleaning time or observing the cleaning process through a window in the chamber have been used to determine the endpoint of remote plasma cleans. Using a fixed-time clean may result in a cleaning process that is either too short or too long for a particular situation because of changes in cleaning efficiency caused by hardware degradation, changes in operator settings, and slight variations in deposit conditions.
From the above, it can be seen that is desirable to have an efficient and thorough remote plasma cleaning process. It is also desirable to provide an endpoint detect method and apparatus to enhance utilization of the substrate processing equipment and to reduce the overetching of chamber components. The endpoint detection method should provide a reliable indication of the end of the cleaning process under a variety of cleaning conditions.
The present invention provides a method and an apparatus for determining the composition of the effluent from a process chamber by forming a plasma in the exhaust stream and measuring the optical emissions from that plasma.
In one embodiment, a plasma is formed in a cell using the effluent from a processing chamber. A window transmits light emitted from the plasma to an optical detector. A filter between the observation window and the detector can be used to tune the detector to a particular wavelength of light. The selected wavelength corresponds to a particular component in the plasma. Additional detectors and filters may be added to monitor the presence of additional plasma components. Relative concentrations of a plasma component can be determined by monitoring a single wavelength.
In another embodiment, the presence of free fluorine is monitored during a chamber cleaning process that utilizes fluorine species created from a cleaning precursor gas in a remote microwave plasma generator. The relative concentration of free fluorine in the exhaust stream, indicated by the intensity of photon emissions from the plasma at a characteristic wavelength of fluorine, increases when the chamber cleaning process is completed and the free fluorine is no longer combining with residue in the chamber. After the change in emissions is detected, the cleaning process is ended.